1. Field of the Invention
This invention relates to a shoulder endoprosthesis comprising a capitulum, a middle section, to which the capitulum is attachable, and a shaft section, which is likewise attachable to the middle section.
2. Prior Art
The replacement of joints which are diseased or damaged through injury has for many years been part of daily practice in accident surgery and orthopaedics. In particular, artificial hip and knee joints have become firmly established in medicine.
In recent years, the replacement of damaged shoulder joints has also increasingly been carried out successfully.
The demands placed on artificial joints derive from the mechanical load placed on them, which occurs as an alternating load and must be borne over many years, as well as from the anatomical conditions to which the artificial joint must adapt, as well as from the environmental influences which the materials used must permanently resist.
Three material categories are basically used today to construct artificial joints. These are corrosion-resistant biotolerant metals; bio-compatible ceramics and plastics which satisfy the pertinent particular special mechanical requirements and the aging behaviour of which guarantees the ability of the joint to perform over many years.
Special problems which occur for joint endoprosthetics as a whole are the particular individual anatomical conditions to which the artificial joint must be adapted in order to reproduce, at least approximately, in the artificial joint the biomechanical and kinematic conditions which are present in the healthy joint.
Depending on the individual conditions, different sizes and designs of artificial joints are therefore manufactured to suit every application if at all possible. This is relatively easy in the case of hip joints, as the hip joint is a ball-and-socket-joint and, consequently is relatively easy to oversee in anatomical, biomechanical and kinematic terms.
The conditions in the knee joint are much more complicated, as the degrees of freedom of the knee joint are clearly limited by the combination of cap and ligament and sliding and rolling movements are superimposed on the natural joint.
Conditions are also relatively complicated in the shoulder joint, as the shoulder joint guarantees an exceptionally large degree of movement and the cavity (scapular glenoid cavity) covers only a small section of the capitulum.
Conditions become particularly difficult in the case of a joint destroyed by injury, as each individual case presents its own special problems. For this reason, shoulder prostheses are normally constructed modularly.
Thus there are many possible combinations which can be matched relatively well to the individual case.
However, this modularity also involves risks, which can primarily consist of reduced endurance or necessitate complicated connection mechanisms which complicate the operation technique and thus also increase the operation times.
A shoulder endoprosthesis with a shaft, a head neck section and a head cap is known from DE-A-195 48 154. The neck section is adjustable vis-à-vis the shaft so that the effective length of the shaft can be changed.
A shoulder endoprosthesis is known from EP-A 0 679 375 in which the middle section is adjustable vis-à-vis the shaft section in its rotation position about the shaft axis. The middle section is not adjustable along the shaft axis and the required length of the prosthesis is achieved by having middle sections of different lengths.
Also in the case of the shoulder endoprosthesis, which is known from FR-A 2 664 809, the length is likewise matched by intermediate pieces of different lengths.
A shoulder prosthesis is described in EP-B1-0 278 807 which consists of an upper arm piece and a piece bearing the ball-and-socket-joint. Both sections are movable lengthways against each other so that anatomical size ratios can be set. The rotation of the two sections against each other is however not fixed. Also anatomical angle ratios of the ball head to the humerus shaft cannot be set.
A shoulder prosthesis is known from DE-T2-38 76 087 which consists of a shaft section and a head section which are connected to each other via a cone plug connector. Very different anatomical conditions resulting from trauma cannot be taken into account with this prosthesis in either the head area or in the shaft area. Also, the angles are not settable.
A shoulder prosthesis is known from EP-B1-0 127 503 which consists of a humerus shaft and a joint cap. Head and shaft are switched in this prosthesis compared with the natural shoulder joint, so that the head section is attached to the shoulder blade. The advantage of the design is that it is luxation-resistant in all anatomical positions. However, this joint can be used only within limits, as it cannot be adapted to the various conditions, in particular in the case of traumatic indications. Moreover, the cavity section is very large and consequently cannot be implanted in anatomically favourable manner in every case.
The objective of the invention is a modularly constructed shoulder prosthesis which suits very differing anatomical conditions, in particular also traumatic indications.
The shoulder endoprosthesis of the invention comprises a capitulum, a middle section, to which the capitulum is attachable, and a shaft section, which is likewise attachable to the middle section, the middle section being adjustable vis-à-vis the shaft section along its shaft axis and in its rotation position about the shaft axis.
Preferably, the middle section has a bore and the shaft section can be clamped fast in this bore by means of a clamping element.
Preferably, the proximal end of the shaft section and the clamping element are provided with co-operating conical surfaces and the shaft section is clamped fast to the middle section by cone wedging.
The proximal end of the shaft section can be constructed as a conical bolt and the clamping element can be a spreadable or expandable sleeve with conical internal surface. By clamping the sleeve onto the conical bolt, the sleeve is expanded so that it is fixed in the bore of the middle section.
Another possibility is to provide the proximal end of the shaft with a conical bore and design it to be spreadable and to clamp a conical bolt into this bore as a clamping element so that the expanded end of the shaft section is fixed in the bore of the middle section.
The overall length of the prosthesis and the rotation angle between middle section and shaft section can be set by positioning the proximal end of the shaft section together with the clamping element that is still loosely seated on it inside the bore of the middle section.
Preferably, the overall length of the prosthesis is set continuously or in steps of less than 5 mm and the rotation angle between the middle section and shaft section is likewise set continuously.
Preferably, the middle section has an element for holding bone fragments. These can be claws or projections for the fixing of bone fragments.
According to the basic concept of the invention, the shoulder endoprosthesis is modularly constructed and consists of the shaft, the clamping element, the middle section and the capitulum. The connection of shaft and middle section can be adjusted continuously or in small steps in the axial direction of the shaft axis. The connection of shaft and middle section can also be continuously adjusted in its rotation angle about the shaft axis (ante- and retroversion angle). The connection of shaft and middle section is capable of absorbing rotation moments. The adjustment of the implant along the shaft axis and the rotation about it take place intraoperatively. These adjustments of the implant are carried out at the implanted shaft.
On the one hand, the sections of the shoulder prosthesis can thus be connected by simple and secure elements which can also still be exchanged during the operation, and on the other hand, the angular position and the effective length can be set continuously.
Through the modular construction, shafts can be used in different lengths and diameters, solid and modular heads with different external geometries can be used and the middle sections can be used with different external geometries. The head and middle sections can also be in one piece.
The shaft section and the middle section can be manufactured from titanium- or cobalt-chromium alloys and the capitulum from a cobalt-chromium alloy or ceramic.